1. Field of the Invention
The present invention relates in general to powder metal blends, and more particularly to a new and improved powder metal blend useful for making an improved engine component such as a valve guide.
2. Description of the Related Art
Recent concerns about the environment have created a renewed interest in the development of the so-called xe2x80x9czero emission enginexe2x80x9d. Ideally, this is an internal combustion engine that does not emit or discharge any pollutants. One source of and a contributing factor to air pollution in an internal combustion engine is the engine lubricant oil which can leak into the combustion chamber from a worn valve stem and valve guide interface. This is the location where the valve is reciprocatingly engaged within the valve guide. Besides being a pollutant itself upon combustion, the leaking lubricant oil containing any sulfur can damage the catalytic converter due to catalyst poisoning and can lead to further air pollution in the form of nitrogen oxides.
The operation cycle of an internal combustion engine is well known in this art. The physical requirements for the intake and exhaust valves, valve guides, and valve seat inserts to effectively interact in sealing the combustion chamber has been studied extensively. It is known that valve seat inserts and valve guides operate under a very harsh environment in terms of mechanical, thermal, and corrosive conditions with the severity depending upon the specific engine application.
In an internal combustion engine, the engine oil is allowed to controllably leak through the valve stem seal to the valve guide for providing lubrication at the valve guide interface. A leakage problem arises with wear and occasionally simply from the operating clearances necessary to accommodate differential heating between the valve stem and the valve guide. Without sufficient operating clearances, the valve stem can overheat and seize or stick within the valve guide.
Meanwhile, consumers still expect more performance from their vehicle""s engines as well as longer and better warranties on the powertrain of a vehicle. As a result, many manufacturers are extending powertrain warranties at least up to 100,000 miles. The automotive industry is constantly seeking improved fuel economy, increased horsepower to weight ratios, lower oil consumption, and better reliability for its automotive engines.
Recent improvements in powder metallurgy have been employed to address requirements for good wear resistance as well as good heat and corrosion resistance along with suitable machinability. Powder metallurgy (P/M) permits latitude in selecting a wide variety of alloy systems as well as offering design flexibility. Additionally, powder metallurgy provides controlled porosity for self-lubrication and facilitates the manufacture of complex or unique shapes at or very close to final dimensions.
P/M valve guides are typically made from relatively low alloy steels containing a ferritic/pearlitic microstructure with solid lubricants such as silicates, free graphite, manganese sulfide, copper sulfide, or molybdenum disulfide. The P/M valve guide is pressed to a low to medium density, sintered using conventional sintering temperatures, i.e., less than about 1150xc2x0 C., and then machined at both ends. An inner bore is formed by reaming. While it is known in this art to oil impregnate valve guides, the impregnated oil is replenished during the operation of the engine. The life expectancy of the valve guides relies on engine oil which lubricates the interface between the valve stem and the valve guide.
The oil leakage problem described previously has heretofore been addressed by attempts to control oil leakage through the valve stem seal by providing a better seal and/or attempts to achieve a compromise between lubricating the valve guide to provide a suitable life expectancy thereof and the undesirable emissions produced from the combustion of the oil into the exhaust system.
There still exists a need for a powder metal blend or mixture for use as a valve guide which can withstand the significantly high temperatures to which the valve stem and valve guide are exposed with little or no lubrication. The powder metal blend must have good thermal conductivity to allow the valve guide to conduct heat away from the valve stem to the surrounding cylinder head to prevent seizure or sticking of the valve stem in the valve guide. The powder metal blend should have superior properties of abrasive and adhesive wear resistance, scuffing resistance, and the ability to run against various types of valve stem materials and valve stem coatings including but not limited to chrome plated and nitrided valve stems.
Accordingly, an object of the present invention is to provide an improved powder metal blend useful for making an engine component.
Another object of the present invention is to provide an improved powder metal blend for making a powder metal valve guide.
Still another object of the present invention is to provide an improved powder metal valve guide particularly suited for operation in an oil starved environment.
Still another object of the present invention is to provide an improved powder metal valve guide with superior thermal conductivity to function as a better heat sink.
Still another object of the present invention is to provide an improved powder metal valve guide which has superior properties of abrasive and adhesive wear resistance, scuffing resistance, and the ability to run against various valve stem materials and valve stem coatings.
Still a further object of the present invention is to provide a powder metal valve guide that prevents valve stem and valve guide from seizure where there is little or no lubricant at the valve stem/valve guide interface.
The above and other objects of the present invention are accomplished with an improved powder metal blend suited for operation in a severe engine environment. The present invention comprises an improved powder metal blend having a chemical composition on a weight percent basis comprising: copper in an amount ranging from about 2 to about 10 percent; a solid lubricant in an amount ranging from about 0.5 to about 5.0 percent; graphite in an amount ranging from about 1.0 to about 3.0 percent; bronze in an amount ranging from about 1.0 to about 8.0 percent; iron and/or copper phosphorus in an amount ranging from about 0.2 to about 1.5 percent; a fugitive lubricant in an amount ranging from about 0.3 to about 1.0 percent; and the balance being a low alloy steel powder containing manganese in an amount ranging from about 0.3 to about 1.0 percent.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying examples, drawings, and descriptive matter in which a preferred embodiment of the invention is illustrated.